U.S. patent application number 12/102517 was filed with the patent office on 2008-08-07 for system and method for design and fabrication of a high frequency transducer.
This patent application is currently assigned to RIVERSIDE RESEARCH INSTITUTE. Invention is credited to Jeffrey A. Ketterling, Frederic L. Lizzi, Mary Lizzi.
Application Number | 20080185937 12/102517 |
Document ID | / |
Family ID | 35424410 |
Filed Date | 2008-08-07 |
United States Patent
Application |
20080185937 |
Kind Code |
A1 |
Ketterling; Jeffrey A. ; et
al. |
August 7, 2008 |
SYSTEM AND METHOD FOR DESIGN AND FABRICATION OF A HIGH FREQUENCY
TRANSDUCER
Abstract
Techniques for fabricating high frequency ultrasound transducers
are provided herein. In one embodiment, the fabrication includes
depositing a copperclad polyimide film, a layer of epoxy on the
copperclad polyimide film, and a polyvinylidene fluoride film on
the epoxy. The assembly of materials are then pressed to bond the
polyvinylidene fluoride film to the copperclad polyimide film and
to form an assembly. The polyvinylidene fluoride film being one
surface and the copperclad polyimide film being the other surface.
The area behind the copperclad polyimide film surface is filled
with a second epoxy, and then cured to form an epoxy plug.
Inventors: |
Ketterling; Jeffrey A.; (New
York, NY) ; Lizzi; Frederic L.; (Tenafly, NJ)
; Lizzi; Mary; (Tenafly, NJ) |
Correspondence
Address: |
KEITH D. NOWAK
CARTER LEDYARD & MILBURN LLP, 2 WALL STREET
NEW YORK
NY
10005
US
|
Assignee: |
RIVERSIDE RESEARCH
INSTITUTE
New York
NY
|
Family ID: |
35424410 |
Appl. No.: |
12/102517 |
Filed: |
April 14, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11136223 |
May 24, 2005 |
7356905 |
|
|
12102517 |
|
|
|
|
60574094 |
May 25, 2004 |
|
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|
Current U.S.
Class: |
310/334 |
Current CPC
Class: |
Y10T 29/42 20150115;
B06B 1/0692 20130101; Y10S 310/80 20130101; Y10T 29/53109 20150115;
Y10T 29/49005 20150115 |
Class at
Publication: |
310/334 |
International
Class: |
H01L 41/16 20060101
H01L041/16; H01L 41/04 20060101 H01L041/04 |
Claims
1. A high frequency ultrasound transducer device, comprising: a
copperclad polyimide film; a layer of epoxy bonded to a first
surface of said copperclad polyimide film; a polyvinylidene
fluoride film bonded to said layer of epoxy on a first side thereof
to thereby form an assembly; and a second epoxy bonded to a second
surface of said copperclad polyimide film surface to fabricate into
said high frequency ultrasound transducer.
2. The device of claim 1, wherein said polyvinylidene fluoride film
bonded to said copperclad polyimide film having a curved shape,
wherein said polyvinylidene fluoride film being a concave surface
thereof and said copperclad polyimide film being a convex surface
thereof.
3. The device of claim 2, wherein said curved shape is spherically
curved.
4. The device of claim 1, wherein an array pattern is formed on
said copperclad polyimide film and said arrays are electronically
connected to transducer electrical traces.
5. The device of claim 4, wherein said array pattern is an annular
array pattern comprising a plurality of annuli.
6. The device of claim 5, wherein said plurality of annuli
comprises five rings.
7. The device of claim 4, wherein printed circuit board traces are
positioned on a printed circuit board and electronically connected
to said transducer electrical traces allowing electronic access to
said array pattern.
8. The device of claim 7, wherein surface inductors are mounted on
said printed circuit board and connected to said printed circuit
board traces for impedance matching.
9. The device of claim 1, wherein one side of said polyvinylidene
fluoride film is coated in gold and acts as a ground plane
10. The device of claim 1, wherein a third epoxy joins a conductive
side of said polyvinylidene fluoride film to a metal cap and metal
connector to form a ground connection.
11. The device of claim 10, wherein said third epoxy is a silver
epoxy.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/136,223, filed on May 24, 2005, entitled "Method of
Fabricating a High Frequency Ultrasound Transducer," which claims
priority to U.S. Provisional Patent Application No. 60/574,094,
filed on May 25, 2004, entitled "Design and Fabrication of a 40-MHZ
Annular Array Transducer," which is hereby incorporated by
reference in its entirety.
FIELD OF INVENTION
[0002] The present invention is directed to design and fabrication
of high frequency ultrasound annular array transducers.
BACKGROUND OF THE INVENTION
[0003] The field of high-frequency ultrasound ("HFU") imaging,
using frequencies above 20 MHz, is growing rapidly as transducer
technologies improve and the cost of high bandwidth electronic
instrumentation decreases. Single element focused transducers,
however, are currently used for most HFU applications. These single
element transducers are limited in their application due to their
inherent small depth of field, which limits the best image
resolution to a small axial range close to the geometric focus of
the transducer.
[0004] HFU transducers primarily utilize single element focused
transducers fabricated with polyvinylidene fluoride ("PVDF")
membranes as their active acoustic layer. These transducers are
relatively simple to fabricate but suffer from a fairly high
two-way insertion loss (.apprxeq.40 dB) because of the material
properties of PVDF. As a result, methods have focused on improving
the insertion loss by optimizing the drive electronics and
electrical matching. Single element PVDF transducers continue to be
the primary transducer choice for HFU applications and have been
fabricated using a ball-bearing compression method.
[0005] Similarly, methods of fabricating single element HFU
transducers using ceramic material have been refined. A number of
ceramic devices have been fabricated successfully to operate in the
HFU regime. Ceramic devices have an inherent advantage over PVDF
based transducers because of their low insertion loss. Ceramic
materials, however, are typically used for flat arrays because they
are difficult to grow or to press into curved shapes. Fabricating
HFU ceramic transducers into concave shapes is known in the art
through the use machining, coating, lapping, laminating and/or heat
forming techniques for bonding and shaping curved transducers.
These known fabrication techniques are used to construct single
element transducers, and are not used to construct an array
transducer.
[0006] Both PVDF and ceramic transducers have been used to great
success for ophthalmic, dermatological, and small animal imaging.
Current methods aim to fabricate individual array elements on the
order of .lamda./2; these small dimensions necessitate advances in
interconnects and electronics to fully implement the technologies.
Accordingly, there exists a need for a technique for the feasible
design and fabrication of a high frequency annular array
transducer.
BRIEF SUMMARY OF THE INVENTION
[0007] It is an object of the present invention to provide a HFU
transducer with large bandwidth, providing fine scale axial
resolution, and small lateral beamwidth, which permits imaging with
resolution on the order of a wavelength. An array transducer
permits electronic focusing that both improves the depth of field
of the device and permits a two-dimensional image to be
constructed, and with a relatively limited number of elements.
[0008] It is a further object of the present invention to
construct, bond, and form a concave annular array transducer out of
an active piezoelectric material, polyimide film, and epoxy using a
ball-bearing compression method.
[0009] It is yet another object of the present invention that the
active piezoelectric material of the transducer can be
polyvinylidene fluoride ("PVDF"). PVDF is an advantageous material
for fabricating high frequency transducers because the material can
be press fit into a curved shape. PVDF also provides a better
acoustic impedance match to water and biological tissue.
[0010] It is a further object of the present invention to
demonstrate the feasibility of a new method to construct PVDF based
annular arrays.
[0011] In order to meet these objects and others that will become
apparent with respect to the disclosure herein, the present
invention provides techniques for fabricating high frequency
ultrasound multiple ring focused annular array transducers. In one
embodiment, the fabrication includes depositing a copperclad
polyimide film, a layer of epoxy on the copperclad polyimide film,
and a PVDF film on the epoxy. The assembly of materials are then
pressed to bond the polyvinylidene fluoride film to the copperclad
polyimide film, and to form an assembly. The PVDF film being one
surface and the copperclad polyimide film being the other surface.
The area behind the copperclad polyimide film surface is filled
with a second epoxy, and then cured to form an epoxy plug.
[0012] Advantageously, the active acoustic element of the
transducer is a PVDF film with one side coated in gold and acting
as the ground plane. A positive array pattern of the transducer is
formed on a copper clad polyimide film ("flex circuit"). The flex
circuit and PVDF are bonded together, press fit into a spherical
shape, and then back filled with epoxy. Transducer performance can
be characterized by measuring pulse/echo response, two-way
insertion loss, electrical cross talk, and the complex electrical
impedance of each array element before and after complex impedance
matching.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0013] FIG. 1 is a schematic diagram which illustrates a positive
array pattern of a high frequency annular array transducer.
[0014] FIG. 2 is an assembly view which illustrates a press fit
device used to assemble a high frequency annular array
transducer.
[0015] FIG. 3 is a plan view which illustrates the electrical
traces and contact pads of the positive array pattern portion of
the high frequency annular array transducer.
[0016] FIG. 4 is a plan view which illustrates electronic access to
the transducer annuli through a customized printed circuit board
connected to the array pattern of the transducer.
[0017] FIG. 5 is an assembly view which illustrates a high
frequency transducer.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Referring to FIG. 1, an exemplary positive array pattern of
a transducer is shown. The circuit patterns are designed as
positive images with a computer-aided design ("CAD") software
package. QuickCAD is used in a preferred embodiment, which is
commercially available from Autodesk Inc. The transducer has an
aperture 110 with a number of equal area rings, known as annuli
140, and separated by a designated annuli spacing 150 between the
annuli 140. In a preferred embodiment, the transducer has a total
aperture of 9 mm with five equal area rings separated by 100 .mu.m
spacings. Transducer electrical traces 155 permit access to each
annulus, and can have the same designated spacing as the annuli
spacing 150 between the annuli 140. In a preferred embodiment, the
electrical traces that permit access to each annulus and the
spacing between the traces are 100 .mu.m.
[0019] From the CAD file, a transparent film with a positive array
image is generated by a commercial offset print shop. This method
of creating the positive image permits line widths and spacings of
smaller than 100 .mu.m.
[0020] The array pattern 100 is formed on a material commonly used
to fabricate flex circuits, such as for example, single sided
copper clad polyimide film. In a preferred embodiment, the single
sided copper clad polyimide film is RFlex 1000L810, which is
commercially available from Rogers Corp. located in Chandler, Ariz.
or any equivalent supplier. In the preferred embodiment, the
polyimide film is 25-.mu.m thick, the copper is 18-.mu.m thick, and
an adhesive layer bonding the copper to the polyimide is 20-.mu.m
thick. Before creating the array pattern 100, the polyimide is
coated with a uniform thickness of positive photoresist, which is
commercially available from Injectorall located in Bohemia, N.Y. or
any equivalent supplier.
[0021] The copper array pattern 100 is fabricated onto the flex
circuit using standard copper etching techniques. In a preferred
embodiment, the positive array image is placed on top of the
photoresist coated polyimide and exposed to ultraviolet ("UV")
light for 2-3 minutes in a UV fluorescent exposure unit, which is
commercially available from AmerGraph located in Sparta, N.J. or
any equivalent supplier. The polyimide is then transferred to a
liquid developer, which removes the photoresist that is exposed to
UV light. The developed film is agitated in a ferric chloride bath
until all the copper in the areas lacking photoresist are etched
away.
[0022] Once the array pattern 100 is fabricated, a microscope can
be used to view the finished array pattern 100 to ensure that the
line widths and spacings between the transducer electrical traces
155 are uniform and of the correct size. After removing the
remaining photoresist, which can be done with steel wool or with
acetone, the array pattern 100 should be tested for electrical
continuity between the annuli 140 and copper contact pads 170. Test
patterns are used to ensure correct line width spacing for both
annuli spacing 150 and transducer electrical traces 155. And in a
preferred embodiment, test patterns are utilized to ensure 100
.mu.m spacing for both the ring separations and line widths.
[0023] Referring to FIG. 2, an annular array transducer is
assembled using a press fit device and layers of material using
compression to bond and form the assembly into a concave shape. In
a preferred embodiment, the press fit device is constructed of
aluminum. The press fit device shown in FIG. 2 uses a base plate
210, a pressure plate 260, and a ball bearing 270 to apply uniform
pressure to a polyvinylidene fluoride ("PVDF") film 230, epoxy 240,
and copperclad polyimide film 250. A top plate 275 presses the ball
bearing 270 into the PVDF 230, epoxy 240, and copperclad polyimide
250 assembly. The base plate 210 has a central hole 220 in which a
tube 215 is inserted. In an preferred embodiment, the tube 215 is
made of Teflon and the ball bearing 270 is made of stainless
steel.
[0024] Assembly of the transducer begins by inserting a tube 215
into a baseplate 210. A polyimide film 250, on which an array
pattern 100 is fabricated, is centered over the tube 215 with the
copper side facing in a direction opposite to that of the base
plate 210, shown facing in the upward direction. An epoxy layer 240
is deposited onto the copperclad polyimide film 250 and array
pattern. As used herein, "epoxy" is understood as including any
resinous bonding agent. In a preferred embodiment, a single drop of
Hysol RE2039 or HD3561 epoxy, which is commercially available from
Loctite Corp. located in Olean, N.Y., is placed onto the array
pattern. A PVDF film 230 is then deposited on the epoxy 240. In a
preferred embodiment, a 4 cm by 4 cm section of PVDF membrane, such
as that commercially available from Ktech Corp. located in
Albuquerque, N. Mex. or any equivalent supplier, is placed over the
epoxy. The PVDF can be 9 .mu.m thick and have one side metallized
with gold, where the metallized side forms a ground plane of the
transducer and should face in a direction opposite to that of the
epoxy 240. A ring 265 is placed over or on top of the PVDF film
230, and clamped with a pressure plate 260. The pressure plate
permits the layers of material to move slightly while also
stretching during the press fit, thus avoiding crinkling of the
films at the edge of the transducer. In a preferred embodiment, the
ring 265 can be made of Teflon.
[0025] A ball bearing 270 is pressed into the PVDF film 230 by
applying pressure to a top plate 275 that is in contact with the
ball bearing 270. In a preferred embodiment, the ball bearing 270
is made of stainless steel and has an outside diameter of 18 mm.
The PVDF film 230 and the copperclad polyimide film 250 are bonded
together with the epoxy 240, and formed to have a spherically
curved shape comprising a concave surface 290 and a convex surface
285. After compression, epoxy is deposited in the tube 215, such
that a plug of epoxy 225 fills the area behind the convex surface
285 of the copperclad polyimide film 250. The assembly can then be
placed into a vacuum chamber to ensure bubbles are not present on
the backside of the copperclad polyimide film 250. In a preferred
embodiment, the press fit device is turned over and the Teflon tube
is filled with epoxy. The whole press fit device is then placed
into a vacuum chamber at approximately 8 Torr. The degassing lasts
as long as necessary to ensure that no bubbles are present on the
backside of the polyimide, which is approximately 40 minutes.
[0026] In an exemplary embodiment, the epoxy plug has an outside
diameter of 13 mm, while the active array has an outside diameter
of 6 mm. The wider epoxy plug ensures a more spherically curved
transducer face and avoids crinkles at the edge of the
transducer.
[0027] After degassing, cure time of the epoxy plug 225 can be
reduced by placing the assembled transducer into an oven. In a
preferred embodiment, after the degassing process the press fit
device is moved into a 40 degree Celsius oven to reduce the epoxy
cure time. When the epoxy cures, the transducer is separated from
the tube 215. The resultant transducer assembly includes an epoxy
plug 225 bonded to the convex surface 285 of the copperclad
polyimide film 250. Referring to FIG. 3, the electrical traces and
their contact pads remain exposed by trimming away any excess
material.
[0028] FIG. 5 illustrates an exemplary embodiment, where an epoxy
510, such as silver epoxy EE129-4 which is commercially available
from Epoxy Technology located in Billerica, Mass. or any equivalent
supplier, is used to join the conductive side of the PVDF film 230
to a ground connection via the metal cap 530 and metal connector
520. The metal cap 530 and metal connector can comprise two
separate units, or be constructed as a single unit. In an
alternative embodiment, the ground connection can also be made by
joining the conductive side of the PVDF film to ground traces on
the polyimide.
[0029] Referring to FIG. 4, in order to electronically access the
annuli 140, a customized printed circuit board ("PCB") 410 can be
fabricated to enable electronic access to the annuli 140 through
the printed circuit board traces 470. The PCB 410 has a connector
420 on one side and a series of smaller connectors 430 on the
opposing side. Cables 440 are connected to each of the smaller
connectors 430. An additional advantage of the PCB 410 is that
surface mount inductors 480 can be soldered directly onto the PCB
410 for impedance matching. The inductors shown in FIG. 4 are
connected in series to the printed circuit board traces 470, but
can also be in parallel to the printed circuit board traces 470. A
mounting bracket made from aluminum rod can hold the transducer 460
and PCB 410. The polyimide film 450 is then wrapped around and
inserted into the connector 420. Thus, the PCB 410 enables
electronic access from the cables 440 to the PCB traces 470 through
a series of connectors 430. The PCB traces 470 are electronically
connected to the transducer electrical traces 155 through a
connector 420. The transducer electrical traces 155 are
electronically connected to the annuli 140.
[0030] In a preferred embodiment, the first connector 420 is a
20-pin zero insertion force ("ZIF") connector, which is
commercially available from Hirose Electric located in Simi Valley,
Calif. or any equivalent supplier. The smaller connectors 430 are
miniature MMCX-BNC connectors, which are commercially available
from Amphenol or any equivalent supplier. The Cables 440 are BNC
cables, such as RG-174 50 Ohms of 0.87 meters length.
[0031] In an exemplary embodiment, prior to applying the press fit
technique described above, an adhesive material such as tape can be
applied to the electrical traces located on the polyimide film.
This prevents the epoxy from adhering to the polyimide films,
allowing the polyimide film to flex after the fabrication process
without breaking the electrical traces. Similarly, an adhesive
material such as tape can be placed on the polyimide traces leading
out to the ZIF connector's contact pads, and removed subsequent to
fabrication. The polyimide film is held in position with an
adhesive material such as tape and centered over the Teflon ring.
The adhesive material is removed after the pressure plate is
secured but before the press fit is applied. Once the top plate is
secured and the ball bearing has been pressed into the assembly,
the screws holding the pressure plate can be loosened. A copper
conductive adhesive material such as copper conductive tape is
positioned on the backside of the PCB in order to form a ground
plane and reduce electrical noise.
[0032] In a preferred embodiment, the results from a piezoelectric
transducer modeling software package, such as PiezoCAD that is
commercially available from Sonic Concepts located in Woodinville,
Wash. or any equivalent supplier, is used to determine the best
impedance matching for maximizing the two-way pulse/echo response.
Based on the model results, an appropriate surface mount inductor
is selected and soldered directly onto the PCB board. The complex
impedance can again be measured to ensure that the reactance at the
center frequency is in fact zero. Impedance matching eliminates the
complex component at a desired frequency for better transducer
efficiency.
[0033] In an exemplary embodiment, a 5-ring annular array
transducer is fabricated with equal area elements and 100 .mu.m
spacing between the annuli. The total transducer aperture is 9 mm
and the radius of curvature is also 9 mm. The inner and outer radii
of the annuli when projected onto a plane are 0, 1.95, 2.05, 2.81,
2.90, 3.47, 3.56, 4.02, 4.11 and 4.50 mm. The projected spacings
between elements can sometimes be slightly less than 100 .mu.m
because the initial pattern is designed as a planar layout and then
press fit into a spherical curvature.
[0034] In an exemplary embodiment, impedance measurements are made
of each annulus in order to determine the most efficient electrical
matching. Based on piezoelectric transducer modeling, the
transducer capacitance is matched with an inductor connected in
parallel and located on the PCB. Parallel inductance is selected
because it results in a larger improvement for the two-way
insertion loss but with a decrease in bandwidth. All of the array
elements can utilize the same matching inductance. When using a
single matching inductance, however, the frequency at which the
matched reactance occurs can vary somewhat for each ring. In a
preferred embodiment, a value of 0.33 .mu.H is calculated as the
best matching at 40 MHz. In the ideal case the reactive component
for each ring should be zero at 40 MHz.
[0035] In an exemplary embodiment, the total transducer aperture
can be 6 mm with a geometric focus of 12 mm. In this embodiment,
the inner and outer radii of the annuli when projected onto a plane
are 0, 1.22, 1.32, 1.8, 1.9, 2.26, 2.36, 2.65, 2.75 and 3.0 mm. In
this arrangement, the transducer capacitance is matched with an
inductor connected in series and located on the PCB. The inductor
value of 0.82 .mu.H is calculated as the best matching at 40
MHz.
[0036] Impedance matching may also increase the pulse/echo response
for the same excitation signal. An increase in pulse/echo
sensitivity can be achieved at the cost of reduced bandwidth.
Impedance matching also improves the two-way insertion loss over
the unmatched case.
[0037] PVDF based annular arrays can be constructed using a copper
clad polyimide film to form the array electrode pattern. After
impedance matching, the performance of the array elements should be
similar to what has been reported for single element PVDF
transducers.
[0038] Those of ordinary skill in the art will appreciate that the
foregoing discussion of certain embodiments and preferred
embodiments are illustrative only, and does not limit the spirit
and scope of the present invention, which is limited only by the
claims set forth below.
* * * * *